The present invention relates to an optical sensor device for the detection of ambient light, which is adapted to be coupled to a pane, in particular to a windshield of a vehicle, according to the generic portion of claim 1.
Sensor devices of this type are made use of mainly as light sensors for controlling the vehicle lighting. The use of traditional lenses for influencing the optical path, such as, e.g., the lenses inclined against the windshield of the rain sensor shown in EP 1 068 112 B1, requires a comparatively large amount of installation space.
Smaller design shapes are possible when holographic structures are used, as known, e.g., from WO 03/026937 A1. These sensors are based on the principle of light diffraction with the aid of diffractive elements and therefore, resulting from this principle, have the disadvantage of a substantially lower useful light efficiency and a higher sensitivity to interfering light.
DE 196 08 648 C1 proposes that in an optical sensor device the light entry and light exit surfaces of the light guide unit be made in the form of Fresnel lenses. Since, however, the surfaces of the light guide in which the lenses are formed are perpendicular to the surface of the pane, the installation space required for this device is very large.
Further fundamental disadvantages of known optical ambient light sensor devices are found in the great manufacturing expenditure and in too narrow a reception range. The attempt to produce an extension in sensor devices having Fresnel lenses in that the light receiver is placed further away from or closer to the focal point of the Fresnel lens fails because the reception characteristic at the same time is split up into one or more further beams of rays far outside the intended reception range. The same holds true for the attempt to increase the receiver surface.
DE 20 2006 005 665 U1 discloses an optical rain sensor device including a cuboid lens plate arranged parallel to the windshield of a motor vehicle. The front of the lens plate facing the transmitter and the receiver of the device includes Fresnel steps which form two aspherical Fresnel lenses the optical axes of which are each inclined by about 45 degrees in relation to the windshield. The first Fresnel lens provides for a substantially parallel orientation of the rays of the light beam radiated by the transmitter, so that the rays are coupled into the windshield in the form of a light beam having a parallel orientation. The second Fresnel lens makes sure that the light beam coupled out of the windshield and still oriented parallel is focused onto the receiver.
DE 20 2007 036 492 A1 shows a directional daylight sensor configured as a generic optical sensor device. On its surface facing the light receiver, the lens plate of the device has Fresnel lens structures, and on the opposite surface facing the pane, it has Fresnel reflector structures. A light beam that impinges parallel on the pane passes through it at an angle and is then coupled into the lens plate through the Fresnel reflector structure and directed perpendicularly through the lens plate onto the Fresnel lens structure and is focused by the latter onto the light receiver. Basically, however, daylight sensors which are based on this Fresnel lens plate technology have a certain sensitivity in further, undesirable receiving directions in addition to the desired principal reception characteristic.